I am planning to fit a Myford Quick Change Gearbox to my 1965 era Super 7. Rather than try and obtain all the Metric and BA gears and use the metric quadrant, I was looking for an approximation gear set. Searching the web I see that John Stevenson is quoted as having described a scheme using a 31 or a 34 gear in place of the 24 tumbler. But I cannot find any more information. I would love to see tables of threads produced and errors, maybe the construction of an excel spreadsheet, and perhaps obtain the two gears.

I am sure this has been discussed endlessly on this site and others but the search here won't find anything, and neither will Google. If I could I would email John Stevenson directly because he seems such a helpful chap.

Ok, I have answered part of my question. I find that the ME home Page Google search works vastly better than the 'Keyword in a Thread in All Topics' search on this page. Perhaps I am doing it wrongly? If you try searching "myford Gearbox" or "myford metric" from both searches you may see what I mean.

From the Google ME search I found the intriguing 'Lady Engineer' thread from 04/09/2011 that had a very helpful reply from John Stevenson and that has pointed me to the metoolsonline site and the 33/34 gear pair with a table. Super. But, I see that the metric approximations claimed include pitches 0.45, 0.75. and 0.90 mm which do not seem very useful to me. I need 0.5, 0.7 and 0.8 mm pitches to cover 3,4 and 5 mm threads respectively and I presume so does everyone else?

Totally agree with John's view that the "quick change gearbox" is nothing of the sort if you need to swap to the metric quadrant.

Last thought – does anyone know whether the tumbler only gear swap enables the production of any BA threads? If I built a spreadsheet I could answer this myself perhaps. I guess there must be a copy out there.

Thank you Gentlemen for the helpful and quick replies,

I now have a bit of reading to catch up on:

1. lathes.co.uk/latheparts/page14 'NthreadsP and BoxfthreadP screwcutting programs'

2. MEW 177 D A G Brown 'Metric Screwcutting'

3. MEW 39 D Dew 'Metric Pitches on an English lathe'

If anyone has tried making a spreadsheet to tabulate the outcomes from the Myford gearbox, from possible gear inputs, I would love to hear. That would have helped me find what John Stevenson kindly pointed out, and feel less of a twit for missing the obvious!

Apologies to Harold for making a blind assumption on how he worked. Clearly the exact Myford gears and quadrant are needed for precise work and gears. I am intrigued by the open thinking behind imperial pitch threads on metric dimensioned items. It is that quality that creates new ideas but I guess it should cause puzzlement for a future engineer who measures a piece of his machinery and studies all the thread charts looking for the right one!

Thanks also to John (who I guess is credited with the tumbler gear change idea?) for leading me to the simple solution I sought. I have myself drawn up a spreadsheet calculating all the metric pitches to be obtained from the Myford gearbox using tumblers from 20 to 35. One error I avoided was not assuming that the Fine Feed numbers printed on the case are accurate: e.g the first and last, 0.0139 and 0.0020, are actually 0.013889 and 0.001984 respectively (shown to 6 DP).

Picking all the best fits from the table, but trying to restrict the number of tumbler alternatives, I drew up a table of tumbler gears and box settings. What interested me was the errors resulting. I reproduce the table and leave others to debate which errors are acceptable and for which tasks.

Pitch Gear Actual Error %

BA

0 1.00 34 0.9995 -0.05%

1 0.90 34 0.8996 -0.04%

2 0.81 20 0.8141 0.51%

3 0.73 25 0.7350 0.68%

4 0.66 30 0.6615 0.23%

5 0.59 21 0.5849 -0.86%

6 0.53 20 0.5292 -0.15%

7 0.48 20 0.4811 0.23%

8 0.43 33 0.4312 0.28%

9 0.39 30 0.3920 0.51%

10 0.35 24 0.3528 0.80%

11 0.31 21 0.3087 -0.42%

12 0.28 24 0.2822 0.79%

Metric Fine

2 0.40 34 0.3998 -0.05%

2.2, 2.5 0.45 24 0.4536 0.80%

3 0.50 34 0.4998 -0.04%

3.5 0.60 25 0.6013 0.22%

4 0.70 25 0.6963 -0.53%

4.5 0.75 34 0.7497 -0.04%

5 0.80 24 0.7938 -0.77%

6 1.00 34 0.9995 -0.05%

8 1.25 33 1.2473 -0.22%

10 1.50 34 1.4993 -0.05%

12 1.75 33 1.7463 -0.21%

16 2.00 34 1.9991 -0.04%

18, 20 2.50 33 2.4946 -0.22%

24, 27 3.00 34 2.9986 -0.05%

I came to the same view as John in that if you have gears 20,21,24,25 and 30 then only 33 and 34 need to be purchased. I have not shown the box settings but I will be most happy to share the spreadsheet for private use with anyone who emails me directly. I have now ordered the gears from metoolsonline.com who are the only source I can find for the 33 gear and thus provide a very valuable service, and at a reasonable price. I have not yet seen their choice of gear set from their table and will be interested to see if it offers anything I have missed with reduced error.

Norm.

Edited By Norman Lorton on 04/12/2012 11:04:14